JPS6326798Y2 - - Google Patents

Description

[Detailed explanation of the idea] This invention relates to a fuel assembly for a nuclear reactor.

A fuel assembly that has been loaded at a predetermined location within a nuclear reactor and burned for a certain period of time is removed from the core and replaced with a new fuel assembly, or is relocated within the reactor core. Since the spacing between adjacent fuel assemblies in the core is narrow, only about 1 mm, during the replacement or repositioning of the fuel assemblies, the support grids of the fuel assemblies or the support grid and the upper or lower nozzle may They may interfere with each other and damage the support grid.

As shown in FIG. 1, the fuel assembly includes an upper nozzle 1 and a lower nozzle 2 arranged vertically apart, and a lattice formed by straps arranged at a predetermined interval between them. A plurality of support grids 3 having spaces are inserted into the grid spaces of these support grids 3 at predetermined intervals and fixed to fixed parts of the support grids 3, and upper and lower ends of each are connected to upper nozzles 1, It has a control rod guide tube 4 connected to the lower nozzle 2, a large number of fuel pellets sealed in a cladding tube, and a pressure spring for these, and is inserted into a predetermined position in the lattice space of the support lattice 3. It is composed of a large number of fuel rods 5 supported by a rigid support part and an elastic support part.

When loading a fuel assembly configured in this manner into a nuclear reactor or taking it out from a nuclear reactor, the fuel assembly A shown in FIG. is in position and stopped, or for fuel assembly A,
As shown in FIG.
interfere with each other, and as a result, a large load is applied to local parts of the support grids 3, 3, and these support grids 3, 3 may be damaged. In FIG. 2, 6 is a fuel assembly positioning pin, and 7 is a lower core plate.

By the way, as a result of investigating examples of damage to conventional support grids 3, approximately half of the total number of fuel assemblies investigated were damaged at the upper end of the uppermost support grid 3 and the lower end of the lowermost support grid 3. It was found that this accounted for an overwhelmingly large amount of damage compared to fuel assemblies where other parts were damaged. As is clear from Fig. 2, when fuel assemblies are loaded into the reactor, the fuel assemblies A' are loaded at the upper end of the uppermost support grid 3 of the already loaded fuel assembly A', and at adjacent or diagonal positions. The lower end of the lowermost support grid 3 of the fuel assembly A interferes with the upper end of the uppermost support grid 3 of the fuel assembly A' and the lower end of the lowermost support grid 3 of the fuel assembly A.
This is due to the fact that the lower end of the tube is easily damaged.

As described above, even though the fuel assembly in which the support grid 3 was found to have been damaged has not completed its original life, it is not allowed to be reused, which is extremely wasteful.

This idea was made in consideration of the above circumstances,
Corner pins are provided at the four corners of the lower surface of the upper nozzle and the upper surface of the lower nozzle so as to protrude from the lower and upper surfaces, and these corner pins are connected to the corner portions of the uppermost and lowermost support grids, thereby making it possible to prevent interference between the support grids of adjacent or diagonally positioned fuel assemblies, or between the support grids and the upper or lower nozzle;
The object of the present invention is to provide a fuel assembly capable of preventing damage to the support grid. One embodiment of the present invention will be described below with reference to Figures 4 to 8. In this embodiment, the same parts as those in the conventional example are given the same reference numerals and their description will be omitted.

In FIG. 4, reference numeral 11 indicates corner pins, which are provided at each of the four corners of the lower surface 1a of the upper nozzle 1 and the upper surface 2a of the lower nozzle 2 so as to protrude from the lower surface 1a and the upper surface 2a.

As shown in FIG. 5, the corner pin 11 is a rod-shaped body (solid pin) consisting of a large diameter portion 12, an intermediate diameter portion 13, and a small diameter portion 14, and the large diameter portion 12 is connected to the upper nozzle 1.
It is welded or screwed into the lower surface 1a perpendicularly to the lower surface 1a, and then its outer periphery is fixed to the lower surface 1a by TIG welding or the like.

Large diameter portion 12 and intermediate diameter portion 1 of corner pin 11
3. The small diameter portions 14 are each formed to have a circular cross section. The outer peripheral surface 12a of the large diameter portion 12 is in contact with the downwardly extending surface of the outer surface 1b of the upper nozzle 1. The length of the large diameter portion 12 located between the lower surface 1a of the upper nozzle 1 and the uppermost support grid 3 is slightly shorter than the distance between the lower surface 1a and the uppermost support grid 3. The outer diameter of the intermediate diameter portion 13 is the same as the outer diameter of the fuel rod 5, and the length of the intermediate diameter portion 13 is slightly longer than the height h of the support grid 3 shown in FIG. This intermediate diameter portion 13 is inserted into the uppermost support grid 3 and is pressed against the rigid support portion 15 of the support grid 3 by the elastic support portion 16 and brought into contact with it. Note that this support grid 3
The width (A in FIG. 5) is the same as or slightly larger than the width of the upper nozzle 1 (B in the same figure).

In addition, along with this, the lengths of the fuel rods 5 at the four corners of the fuel assembly are formed to be shorter than the length of the normal fuel rods 5, as shown in FIGS. 4 and 7, and the upper ends are The lower end is located slightly below the support grid 3, and the lower end is located slightly above the support grid 3 at the lowest stage. Fuel rods 5 at the four corners of these fuel assemblies
A recess 17 is formed at the upper end of each.
The small diameter portion 14 of the corner pin 11 is fitted into the recess 17 . The fitting state of the small diameter portion 14 into the recess 17 is such that the fitting state is maintained reliably without loosening even when the fuel rod 5 and the corner pin 11 vibrate due to the flow of coolant, and the fitting state is maintained securely even when the fuel rod 5 and the corner pin 11 are vibrated due to the flow of coolant. It is also possible to allow the elongation of the fuel rods 5 due to irradiation.

Above, the corner pin 11 on the upper nozzle 1 side has been explained, but the corner pin 11 on the lower nozzle 2 side has been explained.
Since this is also the same as the corner pin 11 on the upper nozzle 1 side, the explanation thereof will be omitted.

However, when loading the fuel assembly A to which the corner pin 11 is attached as described above into the reactor core or taking it out from the reactor core, the space between the upper nozzle 1 and the uppermost support grid 3 and the lower nozzle 2 No other fuel assembly can enter between the support grid 3 and the lowermost support grid 3. Therefore, fuel assemblies in adjacent or diagonal positions are loaded into or taken out of the core in parallel, and interference between the support grids 3 or between the support grids 3 and the upper nozzle 1 or the lower nozzle 2 is avoided. This prevents damage to the support grid 3, the upper nozzle 1, the lower nozzle 2 and the fuel rods 5.

In addition, in the above embodiment, corner pin 1
Although the large diameter portion 12, intermediate diameter portion 13, and small diameter portion 14 of 1 are each made to have a circular cross section, the present invention is not limited to this. For example, the large diameter portion 12 and the intermediate diameter portion 13 may have a rectangular cross section, The portions 14 may each have a circular cross section, the large diameter portion 12 may have a square cross section, and the intermediate diameter portion 13 and the small diameter portion 14 may have a circular cross section. In these cases, the outer surface of the large diameter portion 12 is flush with the outer surface of the upper nozzle 1 or the lower nozzle 2.

In addition, in the embodiment, the corner pin 1
Although 1 is a solid pin, the present invention is not limited to this, and a tubular pin may be used.

In addition, in the embodiment, the corner pin 1
Although the small diameter portion 14 of the fuel rod 1 is fitted into the recess 17 of the fuel rod 5, the present invention is not limited to this, and for example, as shown in FIG. In a fuel assembly in which the rods do not grow by irradiation so much, the fuel rods 5 at the four corners of the fuel assembly are the same as conventional ones, and these fuel rods 5 are attached to the lower surface 1a of the upper nozzle 1 and the lower surface 1a of the lower nozzle 2. The fuel rods 5 are provided at each of the four corners of the upper surface 2a.
Corner pin 2 with a recess 21 formed at the side end
The second recess 21 may be fitted.

As explained above, according to this invention, corner pins protruding from the lower surface and the upper surface are provided at each of the four corners of the lower surface of the upper nozzle and the upper surface of the lower nozzle, and these corner pins are connected to the uppermost and lowermost supporting grids. Since it is connected to the corner part of
When loading or removing fuel assemblies from the reactor core, other fuel assemblies cannot be inserted between the upper nozzle and the uppermost support grid and between the lower nozzle and the lowermost support grid. First, interference between support grids of adjacent or diagonal fuel assemblies or between support grids and upper or lower nozzles is avoided, thereby preventing damage to the support grids, upper nozzles, lower nozzles, and fuel rods. Moreover, since the structure is simple, it is possible to prevent the cost from increasing in order to prevent damage to the support grid, etc.

[Brief explanation of drawings]

Fig. 1 is a front view of a conventional fuel assembly, Fig. 2 is a front view showing the state of interference between support grids of adjacent fuel assemblies, Fig. 3 is a plan view of its main parts, and Fig. 4 is a front view of a conventional fuel assembly.
The figure is a front view showing one embodiment of this invention, FIG.
The figure is a sectional view taken along the - line in Fig. 5, Fig. 7 is a partially omitted sectional view showing the corner pin inserted into the support grid and further fitted into the fuel rod, and Fig. 8 is a sectional view of the other device of this invention. FIG. 2 is a partially cutaway sectional view showing a state in which the corner pin is fitted to a fuel rod, showing an embodiment of the present invention. 1... Upper nozzle, 2... Lower nozzle, 3...
Support grid, 4... Control rod guide tube, 5... Fuel rod,
11, 22...Corner pin, 12...Large diameter part,
13... Intermediate diameter part, 14... Small diameter part, 17, 21
...Concavity.

Claims (1)

[Scope of utility model registration request] In a fuel assembly, a control rod guide tube and a fuel rod are inserted through a plurality of support grids arranged at predetermined intervals, and a lower nozzle and an upper nozzle are attached to both ends of the control rod guide tube, respectively. At each of the four corners of the lower surface of the nozzle and the upper surface of the lower nozzle,
A fuel assembly characterized in that corner pins protruding from the lower surface and the upper surface are provided, and these corner pins are connected to the corner portions of the uppermost and lowermost support grids.